Linear accelerators for charged particles



Oct. 22, 1957 c, w. MILLER ETAL 2,310,855

LINEAR ACCELERATQRS FOR CHARGED PARTICLES Filed April 9, 1954 2Sheets-Sheet 1 Oc 22, 1957 c. w. MILLER ETAL LINEAR IACCELERATORS FORCHARGED PARTICLES Filed April 9, 1954 2 Sheets-Sheet 2 BUNGHER SECTIONTRANSITION SECTION :0 L ///A fi :IZIIIIIII: II U -I //f/////////ATTORNEYS Patented Oct. 22, 1957 LINEAR ACCELERATORS FOR CHARGEDPARTICLES Charles Walter Miller, Sale, England, and Michael CrowleyCrowley-Milling, Colwyn Bay, Wales, assignors to Metropolitan-VickersElectrical 7 Company Limited,

' London, England, a British company Application April 9, 1954, SerialNo. 422,210 Claims priority, application Great Britain April 14, 1953 3Claims. (Cl. SIS-3.6)

' This invention relates to linear accelerators for charged particlesand has an important application in linear accelerators of thetravelling wave type.

A normal linear accelerator of the travelling wave type consists of aninitial section known as the buncher section followed by a main section.

In the buncher section the electrons are collected into relativelycompact bunches which move along with the travelling wave at somepredetermined phase position. At the same time the electron bunches areaccelerated and at the end of this section the velocity will normallyapproach that of light whilst the electron energy will be of the orderof 1 MeV.

During the rest of the travel down the main section of the acceleratorthe bunches of electrons remain relatively compact and may stay at thesame phase position with respect to the wave or may be allowed to driftslowly to the peak of the wave in order to obtain maximum energy.

Important factors affecting the operation of the machine are theeficiency of the bunching and the phase position at which the bunchesride on the waves, the maximum energy output being obtained with thebunch riding on the crest of the wave.

Linear accelerators have therefore, in some cases, been so designed thatbunching has occurred low down in the wave giving efiicient bun chingand the bunches then allowed to ride to the crests so as to give maximumenergy. Alternatively, if constant energy is aimed at, bunching iseffected low down and the bunches then maintained at some point inadvance of the crest, in which case they will tend to adjust their phaseposition automatically if the input power varies so as to tend tominimise variations in output energy for variations in input power.

The main object of the present invention is to provide a linearaccelerator in which the output energy is variable at'will.

According to the present invention a linear accelerator for chargedparticles includes a waveguide along which the electrons are acceleratedby a travelling electromagnetic wave, which waveguide comprises abuncher section followed by a transition section followed by a mainsection whereof the buncher section is so designed that the bunchingoccurs with a phase advance from the wave crests which is dependent uponthe power input, and the transition section is designed to retard theelectron bunches to the trailing slopes of the waves by substantiallyconstant phase angles and the main section is designed to givenegligible phase change of the electron bunches, whereby the distancesof the electron bunches form the wave troughs and hence the energyoutput of the apparatus is dependent upon and controllable by the powerinput.

It will be appreciated that whereas the phase advance of the bunchesleaving the buncher stage is a function of input power, a substantiallyconstant phase retardation will be produced in the transition stageirrespective of the power input.

Preferably, the apparatus is so designed that at full power input thebunches leave the transition section at the crests of the waves and forreduced power input, the bunches will leave the transition section lowerdown the trailing slope of the wave, thus giving a reduced energyoutput.

It will be appreciated that the phase position of the hunches willremain substantially constant along the main section and hence the finalphase position will be substantially the same as that at the output endof the transition section.

Such an arrangement renders the output energy dependent upon the inputpower and hence the output energy may readily be controlled by varyingthe input power.

Reference will now be made to the accompanying drawings, in which:

Fig. 1 illustrates graphically the operation of a known form of linearaccelerator, whilst Figs. 2 and 3 are corresponding graphs illustratingthe operation of apparatus embodying the present invention; and

Fig. 4 shows diagramatically one arrangement of apparatus embodying theinvention.

Referring first to Fig. l, the graph A illustrates the conditionsobtained in a bunching section, whilst graph B shows the conditions atan intermediate section or the early part of the main section, whilstgraph C illustrates the conditions along the major part of the mainsection.

In the apparatus illustrated, the electrons are collected low down onthe wave in the buncher giving efficientcollection, as shown by thelines over the waveform, it being appreciated that whilst only a fewcycles have been illustrated, it is intended that these should beexplanatory of the conditions obtained in this section. i

After bunching is completed, the electrons are allowed to ride up to thepeak as shown by the graph B and they are then maintained at the peakalong the main section of the accelerator as shown by graph C. Such anarrangement is intended to satisfy certain requirements such as maximumcollection of electrons and maximum energy.

In an alternative known arrangement the electrons are bunched as shownby graph A, but after bunching, instead of being allowed to ride to thecrest, they are maintained in advance of the peak (say. at 45 inadvance) throughout the remainder of the linear accelerator.

As above explained this gives a machine whose energy is relativelyindependent of radio frequency power input since in any variation ofpower input the bunching tends to re-position itself on the wave andthis comensates for the change of power input.

As above explained, in arrangements embodying the present invention theelectrons are bunched on the forward slope of the wave and then retardedby a substantially constant amount in a transition section after whichthey remain, during their passage along the main section of thewaveguide, in the final in the transition section.

Fig. 2 illustrates such an arrangement when the linear accelerator issupplied with the full radio frequency power, and in the buncher (asillustrated by graph A) the electrons are bunched at about 30, i. e. inadvance of the peak, and in the transition section which is the sectionof guide whose wave velocity is greater than the position they havereached electron velocity in this part of the path, the electrons" arepermitted to ride up to the peak, and throughout the remainder of theguide, which may have a uniform phase velocity, the electrons (which bythis time have suflicient energy to be regarded as traveling at lightvelocity) will be maintained at the peak of the wave and so give maximumenergy;

If, the power input is reduced, bunching will take place higher up,thewave,,with the result that afterthe bunches hav e;retarded at "thetransition section they'will be .at a

position 'on the trailing slope of the wave, with a result that'theoutput energy will'be reduced. 7 r.

' Fig- 3 Shows an extreme casein which the R. Rinput power is reduced toone quarterof full power, that is, the. R. F. field is reduced to onehalf amplitude.

i It will be appreciated that in the transition section in which thebunch is retarded it will be moved down the trailing slope of the waveto' a'position say 60 behind the namely; an initial bunching section forbunching and peak "and it will remain at' this position throughout theremainder of the accelerator. The field at a point 607 behind the peakis obviously half the peak field, and since,

the peak field'is already half of that'obtained in the full power'condition, the acceleration'given to the electrons in .the main sectionof guide is only one quarter of that obtained in the full power case.Thus, if we neglect the energy received in the buncher and transitionsection as being small compared withthe total, it will be seen that inthis design a change of output energy over a 4:1 range can be obtained.Clearly, whilst Figs. 2 and 3 show e'X-V treme conditions, intermediateconditions can be obtained in which-the bunches down the main sectionoiwave,

travel at positions intermediate these shown in graph C ofFigs. 2 and.3.

Referring now to Fig. 4

consists of a waveguide wall 1 having apertured {diafphragms or plates 2spaced apart along the interior of the waveguide. A waveguide inlet 3 isprovided at'the inlet end and a waveguide outlet 4 at the output end. 71 At the top end of the waveguidesas shown in the drawing there areprovided electron gun means '5 and electrons projected into thewaveguide from the gun means 5 will be accelerated along the waveguideby a travelling electromagnetic waveuntil they strike a'target or window6 at the opposite end.

'This may allow the electrons to pass out through an opening 7 or it mayintercept the electrons sufliciently,

to cause the generation of X-rays.

The invention consists in providing the waveguide with a' bunchersection A, a transition section B, and a main section C in that order.

In the drawing the variations in size both of the waveguide diameter andof the iris diameters have been great- 1y: exaggerated for explanatorypurposes, it being appreciated that in actual operative apparatus thevariations in dimensions are relatively small.

In accordance with the invention, the buncher section A is designed withthe iris diameters progressively increasing and thewaveguide diametersprogressively reducing in such a manner that the electrons will bunch onthe forward slopes of the travelling electromagnetic wave and whichshows one form of apparatus embodying the invention, the apparatusshown' guide structure, means for propagating an electro-magnetic wavealong said waveguide structure was to impart en-' ergy to saidelectrons, said waveguide structure compris ing three consecutivesections, namely; an initial bunching; section for bunching andaccelerating the electrons to a velocity approaching the speed of light,and having means for causing the phase of the bunch relative'to theelectro: magnetic wave to be dependent on the input power of,

with a phase advance from the peak which depends upon 7 the input power;t

In the transition section B, the iris aperture diameters and waveguidediameters both remain substantially constant, the iris aperturediameters being slightly larger than' in the'main section,'the designbeing such as to retard the electrons by a substantially constantamount.

Finally, in the main section C the iris aperture diametei's andwaveguide diameters bothremain substantially fconstant; but the irisaperture diameters are reduced in comparison with those in thetransition section B.

For further information regarding the'de'sign'of such 7 v waveguides,reference may be made to the Proceedings of the Physical SocietyKGreatBritain, l9 4 8, volume 61,

* at page 246);

accelerating the charged particles to a velocity approach ing the speedof light andrhaving means for causingthe phase of the bunch relativetothe electro-magnetic wave to be dependent on the input power of saidwave, a transition section having means'tor retarding the phase of thebunched particles bya constant amount, and a mainsec-v tion having meansfor imparting energy to the bunch at substantially constant velocity byan amount dependent on the phase of the bunch,'whereby the output powerof the charged particlesis dependent on the input powerof theelectro-magnetic wave. 7 I

2. In a linear accelerator for electrons, a waveguide structure, meansfor injecting electrons into said wavesaid Wave, a transitionsectionhaving means for retarding the phase of the bunched electrons bya constant amount,

and a main section having means for imparting energy to the bunch atsubstantially constant velocity by an amount a dependent on the phase ofthe bunch, wherebythe output. powerof the electrons is dependent on theinput power of the electro-magneticWave. r s e 3. In a linearaccelerator for electrons, a waveguide structure, means forinjecting'electrons into said wavegu de structure, means for propagatingelectro -magnetic waves along said waveguide structure so as to impartenergy to said electrons, said waveguide structure com-' prising threeconsecutive sections, namely; an initial bunching section for bunchingthe electrons and having means for causing a phase of, the bunchrelative to the f electro-magnetic waveto be dependent on the inputpower of said wave, said means comprising acircularwaveguide;

of progressively decreasing diameter. and containing spaced iriseshavingapertures of progressively increasing 7 diameter, a transition sectionhaving means for retarding the phase of the bunched electrons by aconstantamount said means comprising a circular waveguide of constantdiameter and containing spaced irises having apertures of constantdiameter, and a main section having means'for' imparting energy to thebunch by an amount dependent V on the phase of the bunch, said meanscomprising a ciri cular waveguide of constant diameter containing spaced1r1ses having apertures reduced in diameter in comparison with theirises in said transition section whereby the output power of theelectrons is dependent on the input power of the electro-magnetic wave.

References Cited in the file of this patent UNITED STATES PATENTS2,521,426 I Trump et al. Sept. 5,1953 2,653,270 Kompfner Sept. 22, 19532,687,777 Warnecke et al. Aug. 31, 1954 FOREZGN PATENTS 969,886 FranceMay 31, 1950

